The present invention relates to a scanned keyboard for entering data and function instructions into remote data processing equipment. The keyboard is formed with a matrix of key switches which are scanned so that any actuated key switch of the keyboard will result in the transmission of a unique code for that key switch. The transmitted code for an actuated key switch occurs at a unique time slot in the keyboard scan cycle.
The construction details of the suitable key switches, or selectors, which may be used in conjunction with the keyboard design of the present invention are disclosed in United States Patent Application, Ser. No. 525,416 filed in the name of Victor M. Bernin, Robert Madland and Carl Knoll on November 20, 1974 and assigned to the assignee of the present invention.
The keyboard of the present invention employs a keyboard encoder array which scans the key switches of the keyboard matrix in successive scan cycles that are appreciably shorter in duration than the fastest anticipated time delay between the manual operation of two successive key switches. Certain errors may occur in the operation of keyboard entry systems during rapid burst typing of familiar groups of letters, such as "and", "the", and the suffix "ing". Construction details of an N-Key Rollover Protection System to eliminate these errors, which uses a shift register and a comparator, may be found in the patent application of Keith A. Engstrom, United States Patent Application, Ser. No. 457,884 filed April 4, 1974, which is assigned to the assignee of the present invention. An N-Key Rollover Portection System allows the transmission of the code for a given key switch upon the depression of that key switch regardless of the state of any other key switch in the keyboard matrix. A key detect signal is generated in the N-Key Rollover Protection System of the prior Engstrom application for each new actuation of a key switch, and thus, the letters t, h and e of the word, "the" will be read out as they are sequentially actuated.
The N-Key Rollover Protection System is constructed in the keyboard of the prior Engstrom application by use of a shift register, which has a number of stages that are equal to the maximum number of key switches in the keyboard matrix, and a comparator. The output of the shift register, which represents the state of a particular key switch during the last scan cycle, is compared with the state of the key switch during the present scan cycle, and the comparator indicates whether or not the same key switch has been actuated during two successive scan cycles. If the key switch has been held down during two successive scan cycles a key detect signal will not be generated for this particular key switch, but it will be generated upon the initial actuation of another key switch, thus providing the desired N-Key Rollover Protection System.
While an N-Key Rollover Protection System is very desirable for keyboards in which burst entry typing may occur, the electronic keyboard industry standard for the prevention of false code errors has generally been the N-Key Lockout/Two Key Rollover System; and some users prefer this type of system for particular applications. N-Key Lockout permits a code for a given key switch to be transmitted from the keyboard only if that key switch is the only key switch of the keyboard which is depressed over a given time. Two-Key Rollover provides that the code for a given key switch will be transmitted only once for each actuation of the key switch. In this type of system the depression time, for example, between the letter t and the letter h during the typing of the word, "the" may overlap; and if the letter e is next actuated so that both of the letters h and e are actuated during the same time period, the letter h may not be transmitted with a N-Key Lockout/Two Key Rollover scheme because there may never be a time when it is the only actuated key switch. An N-Key Lockout/Two Key Rollover System does have the advantage, however, that when more than two coded key switches are actuated in the same scan cycle no key switch code will be transmitted. Both the N-Key Rollover Protection System of the prior mentioned Engstrom application and the N-Key Lockout/Two Key Rollover Protection System of the present application may utilize the same shift register and comparator, if desired, thereby reducing the number of different types of circuit boards required by a keyboard manufacturer who produces both types of protection systems.